Most observations of seismic waves are made either at or very near to the Earth's surface. However, the elastic properties of the Earth close to the measurement surface show some variability. This variability is due to various changes in the petrophysical properties of the Earth and, among them, permeability changes, presence of fractures, presence of fluids in pores, or compaction, diagenesis or metamorphism changes (Toksöz, M. N., Cheng, C. H., and Timur, A., 1976, Velocities of seismic waves in porous rocks: Geophysics, 41, 621–645). It often results in data perturbations of at least a similar magnitude to the target signal.
Thus, before reliable subsurface information can be retrieved from seismic recordings at or very near to the earth's surface, corrections for these effects are required.
Also, it has been shown that variations in near-receiver elastic properties cause the following complications. Firstly, receiver static variations in the data are receiver-to-receiver traveltime anomalies due to the propagation of most of the seismic energy through the heterogeneous shallow structure. Secondly, lateral variations in free-surface reflectivity cause both differences in the amount of reflected and converted energy, and focussing or defocusing of seismic energy. These effects result in amplitude perturbations, especially on horizontal recordings (Kähler, S., and Meissner, R., 1983, Radiation and receiver pattern of shear and compressional waves as a function of Poisson ratio: Geophys. Prosp., 31, 421–435).
Decomposing the recorded wavefields into upgoing and downgoing P and S waves allows an analysis of said recorded wavefields without the effects of any free-surface interaction (Dankbaar, J. W. M., 1985, Separation of P- and S-waves: Geophys. Prosp., 33, 970–986).
However, to perform wavefield decomposition, the free-surface, reflectivity and, hence, local sub-receiver properties, need to be known.